An accumulator in conventional hydraulic systems in this field consists of a tank containing hydraulic fluid such as oil under the pressure of an inert gas blanket such as nitrogen on top of the fluid. It is usually preferred to separate the gas from the oil by a piston having an elastomeric seal around its periphery to prevent entrainment of gas into the oil. A pump is connected in the system to suck low pressure oil from a reservoir connected to the return line and discharge the oil into the bottom of the accumulator at a high flow rate against the top gas pressure, thereby building up the pressure of the oil stored therein. The stored oil at high pressure is connected to the power line of the system for operating the equipment when conditions require, and a sensing device such as a pressure-actuated switch is connected in the line for controlling the pump motor. A hydraulic pressure relief valve is connected to the stored oil in the accumulator to protect the system from excessive pressure in the event of malfunction of the pressure-actuated switch.
It is desirable that the full capacity of the accumulator tank be utilized to store hydraulic fluid under pressure and that the pump motor be shut off when the piston reaches the top of the cylinder. However, conventional pressure-actuated switches have a substantially wide range between make and break connections.
A number of disadvantages have been experienced with accumulators in conventional systems resulting from inaccurate control of the piston as it reaches the top of the accumulator such that the pressure of the oil continues to build up (sometimes referred to as "top out,") or the pump is shut off prematurely before the piston reaches the top.
For example, if the pressure builds up due to top out, a pressure differential is created across the elastomeric seal around the piston, causing seal extrusion and reducing the life of the seal.
Also, if the piston is allowed to top out, the system becomes what may be called a "hard" system, leaving no room for thermal expansion of the hydraulic fluid and causing the pressure relief valve to open. Once the relief valve has opened it may not reseat properly when the system is restored to normal operation, and leakage of the hydraulic fluid may consequently occur.
These problems are aggravated in accumulator systems out-of-doors, as weather conditions may cause the inert gas to shrink and allow the piston to top out, or cause excessive thermal expansion of the hydraulic fluid, resulting in top out.
If the pressure-actuated switch acts to shut off the pump prematurely before the piston reaches the top of the accumulator, a loss of accumulator capacity for storing the desired amount of hydraulic fluid under pressure results. Moreover, due to the wide range between on and off positions if the pressure-actuated switch, a large volume of the fluid stored in the accumulator may be consumed before the switch starts the pump to replenish the fluid.